Doctor advises taking brief shower to avoid gas risk from poor gas water heater 

On Wednesday, December 14, 2016 at 7:22 PM, lopburi3 said:

Point of use gas water heaters have been available here for many decades - I have had several before electric became available.  And they were not even intended to be vented in those days - at least now most will have provision for a vent system.  This is a currently advertised model.  http://mazuma.co.th/product/lpg-water-heater/lpg-10-5cr/

I remember, as a child an LPG  heater for supplying  hot water for the bath and wash basin in my grandmothers house. 

It vented internally. A well serviced LPG heater shouldn't produce any or very little Carbon Monoxide but it will produce substantial quantities of 

CO2 which will deplete the O2 level and you will eventually notice some .breathlessness...however old properties were notoriously drafty ....